Traditional planar antennas that integrate a radiating aperture and feed structure ensure a physical conductive connection between the two subassemblies to provide a current return path for direct current (DC) control and power conditioning signals as well as RF signals to prevent extraneous radiation from the electrical interface from corrupting the radiation patterns of the antenna. Typical feed structures in these types of antennas tend to feed RF energy into the radiating aperture via a corporate feed arrangement or a combined series/parallel arrangement that provides power distribution as well as aperture tapering in the case of passive phased array antennas. These power distribution networks tend to have many RF power dividers and discontinuities that necessitate the use of stringent design criteria to ensure the cascaded performance of the whole feed meets the requirements of the system. In the case of the edge fed radial waveguide feed, the power distribution is handled by the nature of the dilution of the energy about the antenna radius, but still requires the use of careful design principles to accomplish a robust broadband design.
One instantiation of the radial feed antenna used a relatively narrow band approach for launching and terminating the propagating waves as well as in the discontinuity compensation in the layer transitions. In the launch, a quarter-wavelength open transmission line stub was designed to transition from an axial transverse electromagnetic (TEM) mode to a radial TEM mode. The quarter wavelength open stub launch depends on the resonant length of the center conductor to transition from a guided mode to a quasi-radiative mode as if radiating into free space. The resonance of the launch structure is inherently band limited and difficult to extend beyond 20% bandwidth without adding other tuning mechanisms to compensate for the resonance. The free standing probe also limits the average power handling capacity of the launch to roughly 10 watts or less for a standard SubMiniature version A (SMA) center pin. Any heat accumulated at the launch will be dissipated only through radiation or convection, which will be limited due to the surface area of the probe and the air flow within the waveguide cavity. In addition to the launch, the transition from bottom guide to the top slow wave guide uses one capacitive step to offset inductance caused by the 180 degree e-plane bend. While these approaches are standard for waveguide components, to achieve bandwidths in excess of 30%, it is necessary to use less frequency-dependent methods for the mode transitions and the discontinuity compensation.
In other more broadband radial waveguide structures, the broadband approach has been to use continuous taper transitions that have smooth transitions from one mode to another. An example feed of this feed approach is shown in FIGS. 1A and 1B. This approach attaches the center pin of the connector to a fluted transition shorted to the top guide wall. While this approach can achieve broad bandwidths, the fabrication can become difficult due to the complex curves that create these smooth transitions. These transitions usually must be fabricated using a lathe to follow the complex curvature. If further compensation is needed for matching purposes, the continuous curvature offers only the ability to quicken or slow the transition rather than to offer additional features for capacitive or inductive tuning. In addition, the layer transitions are typically accomplished using chamfers, which gives the designer only one knob to adjust to achieve broadband matching.
Development of LCD/glass-based radiating apertures based on dielectric substrates without external metallization layers prevents providing an electrical attachment method similar to the conventional methods described above.
In many conventional phased array antennas, the radiating aperture is built from a machined aluminum housing that acts as a manifold for integrating thermal and climate control channels with structural rigidity and alignment. The advantage of using aluminum for this function is that aluminum is highly conductive at RF and DC and is readily available and well characterized for machining and assembly. Alternatively, some conventional phased arrays utilize printed circuit board (PCB) technology to reduce the amount of “touch labor” involved in antenna assembly while providing design flexibility to the engineer for RF routing and integrated circuit (IC) integration. Both of these manufacturing technologies provide excellent methods with which the assembly of the antenna can be easily grounded to the antenna chassis and RF feed network.